208 REPORTS ON INVESTIGATIONS AND PROJECTS. 



of the observations with the lOO-inch in this way. This would not be pos- 

 sible if the modified Cassegrain were adopted, as no real image is obtained 

 unless the two mirrors are used in combination. Again, it is equally impor- 

 tant that the telescope be available for use, in the ordinary Cassegrain form, 

 with equivalent focal lengths ranging from lOO to 300 feet, and in the coude 

 form, for high dispersion photography of stellar spectra with a long-focus 

 spectrograph. The modified Cassegrain would not be available for either 

 of these purposes. In short, while it appears to be a very promising instru- 

 ment for certain classes of work, it has not the flexibility which may be re- 

 garded as essential in the case of the 100-inch telescope. Moreover, as the 

 tube is to be made in sections, its length can easily be reduced in case it 

 should ever become desirable to transform the 100-inch into a telescope of 

 the modified Cassegrain type. 



Another plan advocated by Mr. Ritchey was the use of a cylindrical wind- 

 screen, open at the top, in place of a dome. During the day the telescope 

 was to be maintained at a constant temperature within a house which could 

 be rolled away at night, thus affording the advantage of observing in the 

 open air. At first sight this arrangement appeared very attractive, but on 

 further consideration it was abandoned in favor of a dome. It was evident 

 that the proposed wind-screen would not protect the telescope from eddy- 

 currents, and that the inner wall and floor of the drum would be heated by 

 the sun in summer and covered with ice and snow in winter. After long 

 study of shelters of various types, a dome of special construction has been 

 decided upon, and the detailed design has been worked out in accordance 

 with our indications by Messrs. D. H. Burnham & Co., of Chicago. This 

 decision was not adopted, however, until temperature measures in the dome 

 of the 60-inch reflector had convinced us that no disturbance of definition 

 need be feared from the heating of the dome covering, provided that suit- 

 able precautions be observed in the design and ventilation of the structure. 



The open wind-shield was at first discarded in favor of a dome lined with 

 cork, protected from the sun by a ventilated sheet-metal screen, and main- 

 tained at the night temperature throughout the day by a large refrigerating 

 plant. In this connection we are under special obligations to Director Strat- 

 ton and Dr. Buckingham, of the U. S. Bureau of Standards, for a valuable 

 report on the problems presented by such a dome, prepared by Dr. Bucking- 

 ham. With sufficient insulation, there appeared to be no difficulty in accom- 

 plishing the desired results, but the great cost of construction and the heavy 

 annual expenditure for power were formidable objections. Fortunately, the 

 success of the plan of controlling the figure of the coelostat and second mir- 

 rors of the 150-foot tower telescope by constant-temperature water-jackets, 

 pointed to a new and simple procedure, involving much less expense, both 

 for first cost and maintenance. It is true that this plan does not provide for 

 maintaining the interior of the dome during the day at the night temperature 

 and some difficulty from heating of the air, perceptible under the finest at- 



